Conventionally, there has been known a method in which dimethyl ether or a mixture of dimethyl ether and methanol is subjected to dehydration reaction by bringing into contact with a zeolite catalyst to convert into lower olefins containing ethylene and propylene.
This method has a problem that, although the dehydration reaction is continuously carried out while a feedstock gas containing dimethyl ether is fed to zeolite, the zeolite catalyst is gradually deactivated (reversible deactivation) because deposited carbonaceous deposits adhere to the pore surface of the zeolite with time and the active sites which effectively act on the reaction are poisoned. Therefore, it is required to repeat the operation of regenerating the deactivated zeolite catalyst to recover the activity.
For this reason, it has been studied to improve the life of a zeolite catalyst by suppressing the deterioration of the catalyst activity with time from the aspect of the production efficiency and cost of the lower olefin.
Here, the problematic deactivation includes a irreversible deactivation as described later other than the above-mentioned reversible deactivation. The reversible deactivation is poisoning of a catalyst active site caused by the accumulation of carbonaceous deposits, which may be regenerated by burning in air. On the one hand, the irreversible deactivation is the disappearance of the active site due to aluminum removal caused by exposure to steam and heat and may not be regenerated because of the irreversible structural change.
In producing lower olefins from methanol and dimethyl ether (DME) in the presence of a zeolite catalyst, it is considered that the reaction sequentially proceeds by the following pathway to form carbonaceous deposits.
Methanol→Dimethyl ether→Olefin→Aromatic Compound→Carbonaceous Deposits
In order to suppress the formation of carbonaceous deposits caused by this reaction and to suppress the deterioration of a catalyst caused by heat damage, the excessive temperature rise of a catalyst bed is preferably prevented and the removal of the heat of reaction is effective. In addition, the removal of the reaction heat is critical from the viewpoint of safe operation of equipment. For this reason, various methods have been proposed in order to reduce the temperature rise of the catalyst bed.
For example, there has been adopted a method in which, before a feedstock gas is introduced into a reactor producing lower olefins, the reaction is divided into two stages by providing a reactor which converts methanol to dimethyl ether in advance, thereby reducing the temperature rise of the catalyst bed. In addition, there has been adopted a method in which a dilution gas is added into the feedstock gas to reduce the temperature rise. For example, in Patent Document 1, there is shown an example in which, as a dilution gas, hydrogen, helium, nitrogen, carbon dioxide or C1-C7 saturated hydrocarbons are added two to 20 times the amount of the methanol raw material.
It is known that, if the partial pressure of a feedstock gas is reduced by diluting the feedstock gas, the temperature rise of the catalyst bed may not only be suppressed but also the resulting olefin may be prevented to sequentially react, thereby sometimes contributing to the yield improvement of the lower olefin. Therefore, there is widely adopted a method of using a large amount of steam as a dilution gas. In a commercial process, a separation process is required to be provided at a later stage of an olefin synthesis reactor. Steam is more easily separated than other dilution gases, and water is produced as a by-product by the dehydration reaction of the feedstock gas and may be recycled for reuse, which is also a reason for requesting the use of steam as a dilution gas. For example, in Patent Document 2, it is described that lower olefins are produced by setting the partial pressure of steam in the feed to 40 to 80% by volume.
However, although the formation of carbonaceous deposits is alleviated and the time to the reversible deactivation of a zeolite catalyst is extended by the addition of steam in a high concentration, there exists a problem that sufficient catalyst activity and life cannot be obtained after regeneration. It is considered that the skeletal aluminum that forms an active site of a catalyst is extracted from the zeolite framework structure in the presence of steam, causing irreproducible deactivation (irreversible deactivation).
Moreover, when steam is further added as a dilution gas in addition to the steam generated by the reactor for converting methanol into dimethyl ether installed at the earlier stage of a reactor for producing lower olefins as mentioned above, a great deal of evaporation energy is required to generate steam, thus decreasing the thermal efficiency of the whole process. Furthermore, since steam generation equipment is required, the equipment configuration becomes complex, raising the cost of process construction. The production method has also a problem that the operation of the equipment becomes complex.
Furthermore, as another method of suppressing the reversible deactivation of a catalyst caused by the formation of carbonaceous deposits, improvement of a catalyst has been studied. For example, there is known a method in which the density of the active site on a catalyst is decreased by increasing the ratio of Si to Al in ZSM-5 or by supporting a basic metal to poison a part of acid sites.
However, even though any of these methods is adopted, it may not prevent the activity deterioration (reversible deactivation) caused by the formation of carbonaceous deposits on the catalyst with time and the catalyst is required to be regenerated by combustion to remove carbon at a fixed interval.
As a technique aiming at prolonging the regeneration cycle of a catalyst, the present applicants have already proposed a method of reducing the amount of accumulated carbon by the addition of carbon dioxide in a feedstock gas containing dimethyl ether (refer to Patent Document 3). It is considered that this method reduces the accumulation of carbonaceous deposits on the catalyst by gasification of the precipitated carbonaceous deposits by carbon dioxide gas. Although this method reduces the formation of carbonaceous deposits on the catalyst without accelerating the irreversible deactivation and accomplishes the prolongation of the regeneration cycle of a catalyst, further technical improvement has been desired.
Under these circumstances, it has been strongly desired to realize a method for producing lower olefins from a raw material containing dimethyl ether, which may extend the time to the reversible deactivation of a zeolite catalyst by suppressing the formation of carbonaceous deposits on the catalyst, may maintain sufficient activity of the catalyst after regeneration for a prolonged period of time with less irreversible deactivation of the catalyst and may produce lower olefins, propylene in particular, with a high yield at a low cost.
In addition, on the other hand, since a great deal of heat generation is accompanied by the reaction of producing lower olefins such as propylene, ethylene and the like from dimethyl ether or methanol by using a catalyst, heat control becomes an important problem from the viewpoint of suppressing the deterioration of a catalyst and the damage caused by heat in constructing a reactor and moreover from the viewpoint of safe operation of the reactor. For this reason, there have been proposed various methods for reducing the temperature rise of a catalyst bed.
For example, in Patent Document 1, there is described a method of distributing heat generation by installing a reactor which converts methanol into dimethyl ether ahead of a hydrocarbon-production reactor. In addition, in Patent Document 4, there is described a method of reducing temperature rise by adding a dilution gas into a feedstock gas. Further, in a production process of lower olefins using a SAPO-34 catalyst, which is a different type of catalyst, fluidized-bed reactor has been used as heat control.
Since the amount of generated heat due to the reaction is proportional to the feed amount of a raw material, it is also effective for the reduction of temperature rise of the catalyst bed to divide feeding of a raw material by dividing a reactor into multiple stages. In Patent Document 5, there is proposed a method in which multiple reactors are used in series and a raw material is divided and fed into each reactor to be reacted in multiple stages, and it is described that the yield of propylene may be increased without using a costly tubular reactor. This method is expected to alleviate the temperature rise per one reactor by using at least two shaft reactors.
Although the reason why the propylene yield is increased is not specifically described in Patent Document 5, in the method described in Patent Document 5, the increase in the propylene yield is considered to be accomplished by reducing the partial pressure of a raw material. When using multiple reactors, parallel installation of the rectors is effective for the reduction of the temperature rise, but the propylene yield in the resulting lower olefin is expected to increase more when the reactors are installed in series because the partial pressure of the raw material in each reactor may be reduced. It is considered that this is attributed to the suppression of consecutive reaction of the resulting lower olefin to aromatics and the like by reducing the partial pressure of the raw material in producing propylene from dimethyl ether as reported in Non-Patent Document 2, for example.
However, the present inventors have obtained and analyzed data using a test apparatus capable of simulating practical operating conditions. As a result, it was found that, under the practical operating conditions, the propylene yield was not sufficiently increased by only carrying out the reaction in multiple stages and reducing the partial pressure of the raw material by dividing the raw material in feeding. For this reason, it has been desired to realize a method for producing lower olefins further more effectively increasing the propylene yield.    Patent Document 1: U.S. Pat. No. 4,083,888    Patent Document 2: Japanese Published Patent Application No. 2003-535069    Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-104912    Patent Document 4: U.S. Pat. No. 4,083,888    Patent Document 5: Japanese Published Patent Application No. 2003-535069    Non-Patent Document 1: Chemical Engineering, 1980 (8) 87.    Non-Patent Document 2: Journal of Catalysis, 56 (1979) 169.
An object of the present invention is to provide a method for producing lower olefins from a raw material containing dimethyl ether, which may produce the lower olefin, propylene in particular, economically and with a high yield by extending the time to the reversible deactivation of a zeolite catalyst by suppressing the formation of carbonaceous deposits on the catalyst and suppressing the irreversible deactivation of the catalyst, may reduce the amount of water to be recycled to increase the thermal efficiency of the process and may accomplish deletion or substantial scale reduction and simplification of operations of facilities related to the recycling of water and steam generation.
In addition, another object of the present invention is to provide a method for effectively improving the propylene yield under practical operating conditions, in the case of producing lower olefins from a feedstock gas containing dimethyl ether.